Cantilever and method of using same to detect features on a surface

ABSTRACT

A microminiature cantilever structure is provided having a cantilever arm with a piezoresistive resistor embedded in at least the fixed end of the cantilever arm. Deflection of the free end of the cantilever arm produces stress in the base of the cantilever. That stress changes the piezoresistive resistor&#39;s resistance at the base of the cantilever in proportion to the cantilever arm&#39;s deflection. Resistance measuring apparatus is coupled to the piezoresistive resistor to measure its resistance and to generate a signal corresponding to the cantilever arm&#39;s deflection. The microminiature cantilever is formed on a semiconductor substrate. A portion of the free end of the cantilever arm is doped to form an electrically separate U-shaped piezoresistive resistor. The U-shaped resistor has two legs oriented parallel to an axis of the semiconductor substrate having a non-zero piezoresistive coefficient. A metal layer is deposited over the semiconductor&#39;s surface and patterned to form an electrical connection between the piezoresistive resistor and a resistance measuring circuit, enabling measurement of the piezoresistive resistor&#39;s resistance. Finally, the semiconductor substrate below said cantilever arm is substantially removed so as to form a cantilevered structure, and a tip is connected to the free end of the cantilever arm to facilitate the structure&#39;s use in an atomic force microscope.

This application is a continuation of application Ser. No. 07/954,695,filed Sep. 30, 1992, now U.S. Pat. No. 5,345,815, which is acontinuation of application Ser. No. 07/638,163, abandoned.

This invention relates to apparatus and methods of formingmicrocantilevers for use in atomic force microscopes and othermicroscope systems.

BACKGROUND OF THE INVENTION

An atomic force microscope (AFM) scans over the surface of a sample.Typically, in the "contacting mode" of operation, a sharp tip is mountedon the end of a cantilever and the tip rides on the surface of a samplewith an extremely light tracking force, on the order of 10⁻⁵ to 10⁻¹⁰ N.Profiles of the surface topography are obtained with extremely highresolution. Images showing the position of individual atoms areroutinely obtained. In a second mode of operation, the tip is held ashort distance, on the order of 5 to 500 Angstroms, from the surface ofa sample and is deflected by various forces between the sample and thetip; such forces include electrostatic, magnetic, and van der Waalsforces.

Atomic force microscopy is capable of imaging conductive as well asinsulating surfaces with atomic resolution. Typical AFM's have asensitivity of 0.1 Angstrom in the measurement of displacement, and aspring constant of about 1 Newton per meter (1 N/m). Further, thecantilever must be mounted so that the cantilever can approach andcontact a sample.

Several methods of detecting the deflection of the cantilever areavailable which have sub-angstrom sensitivity, including vacuumtunneling, optical interferometry, optical beam deflection, andcapacitive techniques. Optically operated AFM's, while very accurate,are more difficult to build and operate than the present inventionbecause several optical components and fine alignments are required.

SUMMARY OF THE INVENTION

In summary, the present invention is a microminiature cantilever armwith a piezoresistive resistor at the fixed (base) end of the cantileverarm for use in imaging surface features of various objects. Deflectionof the free end of the cantilever arm produces stress in the base of thecantilever. That stress changes the piezoresistive resistor's resistanceat the base of the cantilever in proportion to the cantilever arm'sdeflection. Resistance measuring apparatus is coupled to thepiezoresistive resistor to measure its resistance and to generate asignal corresponding to the cantilever arm's deflection.

The microminiature cantilever is formed on a semiconductor substrate.The cantilever arm is doped to form an electrically separate U-shapedpiezoresistive resistor. The U-shaped resistor has two legs orientedparallel to an axis of the semiconductor substrate having a non-zeropiezoresistive coefficient. A metal layer is deposited over thesemiconductor's surface and patterned to form an electrical connectionbetween the piezoresistive resistor and a resistance measuring circuit,enabling measurement of the piezoresistive resistor's resistance. Thesemiconductor substrate below said cantilever arm is substantiallyremoved so as to form a cantilevered structure, and a tip is connectedto the free end of the cantilever arm to facilitate the structure's usein an atomic force microscope.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional objects and features of the invention will be more readilyapparent from the following detailed description and appended claimswhen taken in conjunction with the drawings, in which:

FIG. 1 is a conceptual block diagram of an atomic force microscopeincorporating the present invention.

FIG. 2 is a block diagram of a piezoresistor and a resistancemeasurement circuit.

FIG. 3 is a graph showing the relationship between surface dopantconcentrations in silicon semiconductors and the piezoresistivecoefficient of silicon piezoresistors made from such materials.

FIG. 4 depicts a piezoresistive cantilever beam under a load.

FIG. 5 depicts the layout of a first preferred embodiment of amicrominiature piezoresistive cantilever.

FIG. 6 depicts a cross-sectional view of the microminiaturepiezoresistive cantilever shown in FIG. 5.

FIG. 7 depicts the layout of a second preferred embodiment of amicrominiature piezoresistive cantilever.

FIG. 8 depicts a cross-sectional view of the microminiaturepiezoresistive cantilever shown in FIG. 7.

FIGS. 9-13 depict the process of manufacturing the cantilever arm shownin FIGS. 5 and 6.

FIGS. 14-17 depict the process of manufacturing the cantilever arm shownin FIGS. 7 and 8.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a conceptual diagram of an atomicforce microscope 100 incorporating the present invention. Amicrominiature cantilever arm 102 with a projecting tip 104 at its freeend is used to probe the surface of a sample 110. Prior art atomic forcemicroscopes (AFM's) keep the cantilever 102 stationary while an XYZtranslator stage moves the sample 110 so as to scan the sample'ssurface. This prior art scanning method, in which the cantilever 102 iskept stationary and the sample 110 is moved so as to scan its surface,is required by the optical components of the prior art AFM's, butpresents difficulties when the sample 110 is large.

An advantage of the present invention is that it allows scanning of asurface to be performed by moving the cantilever 102 rather than havingto move the sample. In "constant force operation mode", movement of theXYZ translator/scanner 112 in the Z direction is controlled by aprogrammed microcontroller or computer 114, which uses informationobtained from the cantilever as to the features on the sample's surface.The signal sent to monitor 116 is the same signal which is sent to thetranslator stage. In "constant Z operation mode" the cantilever is notmoved along the Z direction and the information obtained from thecantilever is directly sent to the monitor 116.

Deflection of the cantilever 102 by surface features of the samplechanges the resistance of piezoresistor 120 in the cantilever by anamount that is proportional to the cantilever's deflection. Thepiezoresistor 120 is coupled by metal connector 122 to a resistancemeasurement circuit 124. The resistance of the piezoresistor 120 iscontinuously monitored and measured by the measurement circuit 124.Typically, circuit 124 is a Wheatstone bridge circuit or any otherconventional resistance measuring circuit. The circuit 124 producesmeasurement signals corresponding to the amount of deflection of thecantilever.

Piezoresistivity is the effect by which, when stress is applied to amaterial, its resistance changes. In the case of a cantilever beam benttransversely, one coefficient, the so-called longitudinal piezoresistivecoefficient, is sufficient to describe the piezoresistive properties ofthe beam. Referring to FIG. 2, consider a bar 130 of piezoresistivematerial that is electrically coupled to a resistance measurementcircuit 124. If a stress is applied in the longitudinal direction x,then the resistance of the bar will change according to the expression:##EQU1## where ΔR is the variation in resistance, R₀ is the resistancewith no applied stress, π_(L) is the longitudinal piezoresistivecoefficient, and S_(x) is the stress applied along the direction x. Thevalues of the longitudinal piezoresistive coefficient in silicon aregiven in Table 1 for different crystallographic orientations and forboth p-type and n-type resistors in FIG. 3.

                  TABLE 1                                                         ______________________________________                                        Longitudinal Coefficient                                                      Longitudinal Direction                                                                          π.sub.L n type                                                                        π.sub.L p type                                ______________________________________                                        100                 π.sub.11                                                                            0                                                111               0          2/3π.sub.44                                   110               1/4π.sub.11                                                                           1/2π.sub.44                                   112               1/4π.sub.11                                                                           1/2π.sub.44                                   ______________________________________                                    

Referring to FIG. 4, consider a cantilever beam with a load P applied tothe end of the beam. The upper half of the beam is subjected to tensilestress along its longitudinal direction, while the lower half of thebeam is subjected to compressive stress. As a result, if the resistivityof the beam is uniform, first order variations in resistance will cancelout. Therefore, the resistor 120 must be defined near the surface of thecantilever, as shown in FIG. 1.

The resistor 120 shown in FIG. 1 experiences tensile stress while thebody of the cantilever experiences compressive stress. Note thatdeflection of the free end of the cantilever arm produces stressprimarily in the base of the cantilever. Given the configuration shownin FIG. 1, it can be shown that the variation in resistance of thepiezoresistor 120 is: ##EQU2## where ΔR is the variation in resistance,R₀ is the resistance with no applied stress, π_(L) is the longitudinalpiezoresistive coefficient, P is the load on the cantilever, L is thelength of the cantilever, H is its thickness and W is its width.

To relate the load P to deflection ΔZ of the cantilever, one mustdetermine the spring constant K of the cantilever: ##EQU3## where E isthe Young's modulus and AZ is the deflection of cantilever. Combiningthe above equations, one obtains the following relationship betweendeflection of the cantilever (ΔZ) and change in the resistance of thepiezoresistor 120: ##EQU4## Thus, as expected, changes in resistance aredirectly proportional to the amount of deflection ΔZ of the cantilever.

FIG. 5 depicts the layout of a first preferred embodiment of amicrominiature piezoresistive cantilever and FIG. 6 shows across-sectional view of the same cantilever. The cantilever arm 102 isconnected at one end to a base 150. The base 150 is a portion of asemiconductor substrate 152. As shown, the cantilever arm 102 is aU-shaped structure having two piezoresistive legs 154 and 156 orientedalong a <100>axis of the silicon crystal in which the cantilever arm 102was formed. A U-shaped piezoresistor 160 is used so that it can beeasily connected to a resistance measuring circuit 124 via a pair ofmetal connection lines 162 and 164. Note that the metal connection lines162 and 164 are supported by the base 150 and therefore do not bend whenthe cantilever arm 102 bends.

The piezoresistor 160 on the cantilever arm 102 is an N+type regionformed by an Arsenic implant. Looking at Table 1, it can be seen thatthe longitudinal piezoresistive coefficient for N-type silicon in the<100>direction is equal to π₁₁ (see FIG. 3). A cantilever with apiezoresistor parallel to the <100>axis maximizes the sensitivity of thepiezoresistor to deflection of the cantilever (because it has thelargest piezoresistive coefficient). In the preferred embodiments, thetwo arms of the piezoresistor 160 in the cantilever arm 102 are orientedparallel to a crystallographic (e.g., silicon crystal) axis having anon-zero piezoresistive coefficient. More generally, the direction oflongitudinal stress when the cantilever arm 102 is bent must have anon-zero piezoresistive coefficient.

Above the N-type silicon substrate 152 is a P+ region 166, in which theN+ piezoresistor 160 has been formed. Oxide layer 1 70 protects thepiezoresistor 160. Metal connections 162 and 164 sit above nitride layer172 and top oxide layer 174. As shown, the metal contacts thepiezoresistor 160 through a contact hole in the oxide-nitride-oxidesandwich structure 174-172-170.

Block 180 is a portion of the silicon wafer that is discarded aftercompletion of the manufacturing process.

FIGS. 7 and 8 depict the layout and cross-section of a second preferredembodiment of a microminiature piezoresistive cantilever. This secondembodiment of the invention is formed using a silicon-on-insulator (SOl)substrate 200. Primary advantages of this embodiment are (1) thecantilever arm 202 is parallel to the base of the cantilever, whichmakes the cantilever easier to use, and (2) there are fewer processingsteps required to manufacture this cantilever.

In this embodiment the two piezoresistive legs 212 and 214 on thecantilever arm 202 are formed from P-type silicon, and are orientedparallel to a <110>axis of the silicon crystal. This embodiment needsonly one protective or insulating layer 216 above the substrate 200. TheP- substrate 220 above the insulator layer 222 in the preferredembodiment is about 1.5 microns deep, and the piezoresistor itself(boron implant region 224) has a depth of about 0.4 microns. Theprotective oxide 216 above the piezoresistor typically has a thicknessof 1000 to 2000 Angstroms (i.e., 0.1 to 0.2 microns). Finally, theinsulator layer 222 typically has a thickness of about 1.0 micron.

Resonance Frequency of Cantilever. When making such cantilevers, theresonance frequency of the cantilever should be higher than 10 KHz toallow fast imaging (i.e., the cantilever must quickly follow thetopography of the sample).

For a cantilever of length L, height H, Young's modulus E and density p,the resonance frequency Fr is: ##EQU5##

For silicon, Young's modulus is E =1.9×10¹¹ N/m² and the density isρ=2.3×10³ kg/m³. Using a cantilever thickness of 0.5 μm, any cantileverlength of up to about 100 μm will result in a resonance frequency above100 KHz.

Sensitivity/Noise. A sensitivity of 1 angstrom in a 1 KHz bandwidth isrequired to demonstrate atomic resolution on some surfaces. Thesensitivity of the cantilevers of the present invention is limited bythe signal to noise ratio in the resistor, given by the Johnson noise Vnas follows:

    Vn=(4k.sub.B TR.sub.0 Δf).sup.0.5

where k_(B) is Boltzman's constant, T is the temperature, R₀ is theresistance of the cantilever when no stress is applied, and Δf is thebandwidth of measurement. The 1 angstrom sensitivity requirement turnsout to be possible, but requires strict limitations on the dimensions ofthe cantilever. Note that R₀ is proportional to the length of thecantilever L and is inversely proportion to its width W. Designparameters (i.e., H, L, W, R₀) must be selected to maximize ΔR/R₀ for agiven spring constant of the cantilever and displacement.

CANTILEVER MANUFACTURING PROCESS

Referring to FIG. 9, the cantilever arm of FIGS. 5 and 6 is manufacturedas follows. The starting material is a <100>n-type Phosphorus doped,10-20 Ωcm silicon wafer 152, herein called the substrate. The first stepis to implant Boron ions into the entire surface of the substrate (e.g.,with energy 180 KeV, dose 6×10¹⁵) So as to generate a P+ region 166 thatis about 0.7 microns deep. Then layers (e.g., 4000 Å) of low temperaturesilicon oxide 168 and 248 are deposited on both sides (top and bottom)of the entire wafer.

The next step is a double photolithography step, one on the top side andone on the bottom side, and therefore top side to bottom side alignmentis required. Photoresist is deposited on both sides of the wafer, topand bottom masks are aligned, and then the wafer is inserted between thealigned masks. Both masks define regions where the deposited oxidelayers 168 and 248 will be removed. The top mask also defines whereArsenic will be implanted. The photoresist on both sides is exposed anddeveloped, and then an oxide etch is performed to produce the profileshown in FIG. 9.

Referring to FIG. 10, the next step is to implant Arsenic through thewindow in the top layer of oxide 248 to create a N+region 250 that isabout 0.3 microns deep. Next, another mask is defined by applying andthen exposing photoresist using a mask that defines the boundaries ofthe cantilever. Then uncovered portions of the oxide layer 248 areetched, followed by etching 1.5 microns of silicon below the etchedportions of the oxide layer to produce a notch 252 projecting into thesubstrate 152.

Next the photoresist is removed using conventional techniques, newphotoresist is applied to the bottom side of the wafer, and then all theoxide on the top side of the wafer is etched away to produce the profileshown in FIG. 10.

A brief annealing step is used to repair implant damage and to activatethe implanted carriers.

Referring to FIG. 11, next, a sandwich of three layers is deposited.First a layer 260 of about 700 Å of low temperature silicon oxide isdeposited on the top side of the wafer, followed by about 1100 Å of lowstress silicon nitride 262 which are deposited on both sides of thewafer, followed by 1 micron of low temperature silicon oxide 264 alsodeposited on both sides of the wafer. The sandwich structure is designedso that it is possible to preserve a thin layer of silicon oxide on topof the cantilever for protection.

The oxide-nitride-oxide sandwich structure on the top side of the waferis patterned and etched using standard photolithography and etchingsteps to produce a contact hole 270 as shown in FIG. 11.

Referring to FIG. 12, another photoresist layer is applied and developedon the top side of the wafer, with a protective photoresist applied onthe bottom side of the wafer, followed by a long oxide etch and then anitride etch to remove the top oxide layer 264 and the underlyingnitride layer 262 in region 272. After removing all photoresist, aprotective photoresist is applied to the top side of the wafer, followedby oxide and nitride etches of the oxide and nitride layers 264 and 262on the bottom side of the wafer. A cross section of the wafer at thispoint is shown in FIG. 12.

Referring to FIG. 13, a metal layer 266 is sputtered onto the wafer andpatterned using standard photolithography techniques to produce metalconnections to the piezoresistor. Then the top side of the wafer iscoated with polyimide and the silicon substrate is wet etched from theback side until all silicon below the boron implant region 166 has beenremoved. The wet silicon etching process automatically stops when itencounters a boron concentration of around 7×10¹⁹ cm⁻³. In the firstpreferred embodiment, this concentration of boron is found at a depth ofabout 0.6 microns. The resulting cross section of the wafer is shown inFIG. 6.

When wet etching the silicon wafer from its bottom side it is onlypossible to open windows which are oriented along <110> directions. As aresult, the cantilever in this embodiment must be at a 45 degree anglewith respect to the opening.

Process for Manufacturing. Second Preferred Embodiment.

Referring to FIGS. 14 through 17, the process for making the secondpreferred embodiment of a piezoresistive microminiature cantilever, asshown in FIGS. 7 and 8, is as follows. The starting material is asilicon-on-insulator wafer 200 with a 1.5 micron thick <100> p-typesilicon layer 220 above the insulator layer 222. The thickness of thetop silicon layer 220 determines the thickness of the cantilever. Boththe top silicon layer 200 and the substrate 200 are <100>oriented. Thefirst step of the manufacturing process is to grow an oxide on both thefront and back sides of the wafer, to strip it off the front side and toblanket implant boron on the front side to a depth of about 0.4 micronsto create P+ region 224. FIG. 14 shows a cross section of the wafer atthis stage of the process.

The next step is a double photolithography step, one on the front sideand one on the back side of the wafer. On the back or bottom side alarge window is opened in the oxide 230. On the top side, usingphotoresist as a mask, the top silicon layer 220 is etched down to theinsulating oxide layer 222. FIG. 15 shows a cross section of the waferafter removing the photoresist.

A thin oxide layer 216 is grown or deposited on the wafer, followed by aphotolithography step, which opens contact hole 232 in that oxide layer216, as shown in FIG. 16.

At this point metal, typically aluminum/silicon, is sputtered onto thefront side of the wafer. Using a fourth and last lithography step, metallines are defined to connect the cantilever resistor to pads. At thispoint the cross section of the wafer is shown in FIG. 17.

Finally, the top side of the wafer is coated with polyimide and thesilicon substrate is wet etched from the back side using a preferentialsilicon etchant solution. The properties of such a solution are that itetches <100>Si planes much faster than <111>planes and that it etchesoxide at a negligible rate. At completion of the silicon etch, oxide isremoved from the back side of the wafer, including exposed portions ofthe insulator layer 222, and polyimide is stripped from the front sideto produce the cross section shown in FIG. 8.

While the present invention has been described with reference to a fewspecific embodiments, the description is illustrative of the inventionand is not to be construed as limiting the invention. Variousmodifications may occur to those skilled in the art without departingfrom the true-spirit and scope of the invention as defined by theappended claims. In general, piezoresistive cantilevers in accordancewith the present invention, suitable for use in AFMs and profilometers,will have a low spring constant (e.g., less than 10 N/m), so that thecantilever will not deform the surface being scanned, and will have asensitivity ranging between 0.1 and 100 Angstroms, depending on theapplication.

What is claimed is:
 1. A cantilever structure for use in an atomic forcemicroscope, the cantilever structure comprising:a cantilever arm havinga fixed end and a free end and a piezoresistive element included withinthe cantilever arm, deflection of the free end of the cantilever armchanging the resistance of the piezoresistive element; a tip projectingfrom the cantilever arm near the free end thereof; the cantilever armhaving a mechanical resonant frequency greater than 10 KHz.
 2. Thecantilever structure of claim 1 wherein the cantilever arm comprises asemiconductor material and the piezoresistive element comprises a dopedregion within the semiconductor material.
 3. The cantilever structure ofclaim 2 wherein the semiconductor material comprises silicon.
 4. Amethod of detecting features on the surface of a sample, the methodcomprising the following steps:(a) providing a cantilever arm, thecantilever arm having a resonant frequency greater than 10 KHz andincluding a piezoresistive element and a tip, the tip being located neara free end of the cantilever arm; (b) bringing the cantilever arm andsample together, until the tip of the cantilever arm is within 500 Å orless of the surface of the sample; (c) moving the sample and thecantilever arm with respect to each other in a direction generallyparallel to the surface of the sample; and (d) detecting changes in theresistance of the piezoresistive element as the cantilever is deflectedin response to features of the surface of the sample.
 5. The method ofclaim 4 wherein the tip of the cantilever is in contact with the surfaceof the sample during step (c).
 6. A method of detecting features on thesurface of a sample, the method comprising the following steps:(a)providing a cantilever arm, the cantilever arm having a resonantfrequency greater than 10 KHz and including a piezoresistive element anda tip, the tip being located near a free end of the cantilever arm; (b)bringing the cantilever arm and sample together, until the tip of thecantilever arm is within 500 Å or less of the surface of the sample; (c)moving the sample and the cantilever arm with respect to each other in adirection generally parallel to the surface of the sample, the tip ofthe cantilever being held at a distance greater than 5 Å from thesample; and (d) detecting changes in the resistance of thepiezoresistive element as the cantilever is deflected in response tofeatures of the surface of the sample.
 7. A method of detecting featureson the surface of a sample, the method comprising the followingsteps:(a) providing a cantilever arm, the cantilever arm having aresonant frequency greater than 10 KHz and including a piezoresistiveelement and a tip, the tip being located near a free end of thecantilever arm; (b) bringing the cantilever arm and sample together,until the tip of the cantilever arm is within 500 Å or less of thesurface of the sample; (c) moving the sample and the cantilever arm withrespect to each other in a direction generally parallel to the surfaceof the sample; (d) detecting changes in the resistance of thepiezoresistive element as the cantilever is deflected in response tofeatures of the surface of the sample; and (e) varying the distancebetween the cantilever and the surface in response to changes in theresistance of the piezoresistive element.